Install trakt plugin on both, for example
That's actually a good idea in theory but in reality both plugins don't 'Pull' the data frequently enough for it to work in practice. What would be excellent is a plugin for MePo that uses' plex's API to change the flags and then one for plex that changes mepo's but the chances of those things happening are slim. Plus there's the whole aspect of relying on the third party solution of trackt, which would be I suppose acceptable if it worked better.